Wireless electronic keys have been getting widely used as door keys of automobiles and houses, engine start keys, etc. For example, in the case of electronic keys for doors, electronic authentication keys carried by humans receive low-frequency request signals from door key apparatuses, and transmit response signals at UHF (ultra-high frequency), so that the door key apparatuses receiving the UHF signals conduct the authentication of IDs. In immobilizers conducting the authentication of engine start, etc., the authentication of Ids is conducted by LF (low frequency) communications. Low frequencies used for transmitting and receiving signals of such electronic keys include not only LF (low frequency), but also VLF (very low frequency) and MF (middle frequency).
Low-frequency-signal-receiving antennas contained in electronic keys for authentication are mainly antennas having coils wound around soft magnetic cores, which exhibit insufficient performance of transmission and receiving depending on the direction because of their directivity. To efficiently detect electromagnetic waves in any three-dimensional directions with reduced directivity, three-axis antennas comprising an X-axis coil, a Y-axis coil and a Z-axis coil in combination are used for electronic keys for authentication.
JP 2004-015168 A discloses, as shown in FIGS. 24(a)-24(d), a non-directional receiving antenna comprising a disc-shaped, soft magnetic core 300 having first to third grooves 301, 302, 303, and an X-axis coil 311, a Y-axis coil 312 and a Z-axis coil 313 successively wound around the first to third grooves 301, 302, 303. JP 2004-015168 A also discloses, as shown in FIGS. 24(e) and 24(f), a core comprising a disc-shaped, soft magnetic core piece 330 having first and second grooves 331, 332 around which an X-axis coil and a Y-axis coil are wound, and a ring-shaped, soft magnetic core piece 340 having a third groove 343 around which a Z-axis coil is wound. Because these cores are formed by one or two core pieces, they can be easily miniaturized with a reduced number of parts. However, because the integral, disc-shaped, soft magnetic core 300 shown in FIGS. 24(a) to 24(d) has a complicated shape with grooves extending in three directions, it cannot be produced by pressing. This is true of the combined cores shown in FIGS. 24(e) and 24(f). In addition, the receiving antenna of JP 2004-015168 A having no bobbin fails to be integrally provided with terminal members. The direct bonding of terminal members to the core fails to achieve sufficient adhesion strength, and the core may be broken under stress.
JP 2007-151154 A discloses, as shown in FIG. 25, a three-axis antenna comprising a cruciform casing 400, a pair of core pieces 421, 422 disposed in a cruciform recess 410 of the casing 400, a pair of X-axis coils 431 wound around one core piece 421, a pair of Y-axis coils 432 wound around the other core piece 422, and a Z-axis coil 433 wound around the cruciform casing 400. However, because this three-axis antenna has a structure in which both core pieces 421, 422 are contained in the cruciform casing 400, a core piece volume per the installation area of the antenna cannot be sufficiently large, resulting in insufficient receiving sensitivity. Also, because the core piece 421 around which the X-axis coil 431 is wound and the core piece 422 around which the Y-axis coil 432 is wound are overlapping each other in the cruciform casing 400, this three-axis antenna cannot be made thinner.